1. Field of the Invention
The present invention generally relates to a carrier recovery apparatus of a vestigial sideband VSB receiver, and more particularly, to a carrier recovery apparatus of a VSB receiver recovering a carrier based on a pilot tone of a pilot included in a broadcasting signal to recover the VSB digital broadcasting signal.
2. Description of the Related Art
At present, there are two modulation systems for a digital broadcasting signal. One is a vestigial sideband (VSB) modulation system, and the other is a coded orthogonal frequency division multiplexing (COFDM) modulation system. The VSB modulation system transmits broadcasting signals at a single carrier while the COFDM modulation system multi-divides and transmits the broadcasting signals through a multi-channel. Currently, the VSB modulation system is mainly adopted in various countries like Korea and the U.S.A. while the COFDM modulation system is mainly adopted in many European countries.
FIG. 1 is a block diagram of a conventional digital broadcasting receiver receiving a VSB analog broadcasting signal that is modulated by a conventional VSB modulation system and transmitted. As shown in FIG. 1, the digital broadcasting receiver includes an analog-to-digital converter (ADC) 10, an I/Q splitter 12, an interpolator 14, a symbol timing recovery (STR) unit 16, a digital frequency phase locked loop (DFPLL) 20, a matched filter 30, an equalizer 32 and a phase tracking loop (PTL) 34.
The ADC 10 converts the received VSB analog broadcasting signal into a digital signal. The I/Q splitter 12 shifts the digital signal to a baseband signal to separate the baseband signal into an inphase (I) channel signal (I-signal) and a quadrature (Q) channel signal (Q-signal). The I-signal and Q-signal constitute a VSB broadcasting signal.
The interpolator 14 compensates for a sampling offset of the VSB broadcasting signal that has occurred during the conversion of the VSB analog broadcasting signal into the digital signal at the ADC 10. The interpolator 14 also detects clock information of the VSB broadcasting signal of which sampling offset is compensated. The STR unit 16 detects timing error information that has occurred with respect to a symbol of the VSB broadcasting signal during the conversion of VSB analog broadcasting signal into the digital signal. The STR unit 16 provides the interpolator 14 with the detected timing error information with respect to the symbol. Accordingly, the interpolator 14 compensates for the sampling offset of the VSB broadcasting signal based on the timing error information provided by the STR unit 16.
The DFPLL 20 recovers a carrier of the VSB broadcasting signal through the detected clock information and a pilot contained in the VSB broadcasting signal. That is, the DFPLL 20 recovers a frequency offset and phase distortions occurring in carriers of the VSB broadcasting signal. After the frequency offset and the phase distortions are recovered, the carriers are multiplied by the I-signal and Q-signal from the interpolator 14 at the multiplier 18 and then output to the matched filter 30.
The matched filter 30 combines the I-signal and Q-signal from the multiplier 18 to the VSB broadcasting signal and filters the VSB broadcasting signal so that a ratio of signal per noise of the combined VSB broadcasting signal can be maximized. The equalizing unit 32 compensates for an error occurred in a channel via which the VSB broadcasting signal is transmitted. The PTL 34 compensates for a remaining phase error in the VSB broadcasting signal.
FIG. 2 is a block diagram showing the DFPLL 20 of FIG. 1 in greater detail. The DFPLL 20 includes a first low pass filter (LPF) 21, a limiter 23, a second multiplier 25, a second LPF 27, and a numerically-controlled oscillator (NCO) 29.
The first LPF 21 indicates the I-signal, which is changed according to the frequency offset of the carrier, by using a pilot tone of the pilot contained in the I-signal of the VSB broadcasting signal that is shifted to a baseband by the I/Q splitter 12. If the frequency offset of the I-signal is positive (+), a phase change of −90 occurs in the carrier. If the frequency offset of the I-signal is negative (−), the phase change of +90 occurs in the carrier. The limiter 23 limits a size of the I-signal output from the first LPF 21. Accordingly, the limiter 23 outputs a pulse wave (limited size of the I-signal) of either value +1 or value −1 from the I-signal output from the first LPF 21. The multiplier 25 multiplies the pulse wave output from the limiter 23 by the Q-signal of the VSB broadcasting signal that is shifted to the baseband by the I/Q splitter 12. The second LPF 27 extracts a DC property from a product output from the multiplier 25. The DC property from the second LPF 27 indicates the frequency error of the carrier. The NCO 29 generates a complex carrier corresponding to the DC property extracted from the second LPF 27 and provides the multiplier 18 with the generated complex carrier. Accordingly, the multiplier 18 multiplies each of the I-signal and Q-signal from the interpolator 14 by the generated complex carrier from the NCO 29 and outputs the I-signal and Q-signal after the sampling offset of the carrier is recovered.
In the conventional VSB broadcasting signal receiver, however, a performance of the DFPLL 20 rapidly deteriorates when the pilot tone is damaged. This is because, a terrestrial wave transmitting a channel for the VSB broadcasting signal is usually transmitted through a multi-path transmitting channel, frequency responses of the transmitting channel include many ‘nulls’. Accordingly, if a frequency band of even one pilot tone matches with one of the nulls, a damage of the pilot tone becomes worse compared to the received VSB analog broadcasting signal. As positions of the nulls of the received VSB analog broadcasting signals are irregular under a variable channel environment, the conventional VSB broadcasting signal receiver usually has an inaccurate recovery of the carrier through the DFPLL 20. More specifically, because the conventional DFPLL 20 recovers the carrier based on the pilot tone, a separate process like tracking has to be performed when the pilot disappears due to a noise and an error. If the carrier is not found even by the tracking, the recovery of the carrier becomes impossible.